I might suggest that you don't mock it, in the spirit of not mocking things you don't own. What I might suggest instead (from the above link):
The prescription implied by "don't mock what you don't own" is to introduce your own shim/wrapper/adapter around it. This effectively cordons off the dependency to a single place in your codebase and contextualizes it in the consistent and easy-to-use style you're trying to promote within your codebase. If there's anything awkward about how one needs to invoke the dependency (maybe a chaining API, multi-step invocation, repetitive default configuration, etc.), it can be swept under the rug into that common adapter.
This feels like an instance where the humble object pattern might be appropriate.